Because of their light weight, aluminum alloys have found wide use in military applications, including military vehicles such as personnel carriers. The light weight of aluminum allows for improved performance and ease of transporting equipment, including air transport of military vehicles. In some vehicles, it is advisable to provide shielding or protection against assault, such as by providing armor plate to protect the occupants of the vehicle. Aluminum has enjoyed substantial use as such armor plate, military specifications pertaining to certain aluminum alloys for armor plate applications applying thereto.
Basically, the requirements for aluminum alloy armor plate are resistance to projectiles, good corrosion resistance, and, in some applications, good weldability. Ballistics tests are often conducted with armor-piercing projectiles such as 0.30 caliber and with fragment-simulating projectiles such as the common 20 millimeter projectile. Obviously, aluminum alloys which satisfy all the requirements for armor plate are desirable, and these desires have been met to varying degrees.
Aluminum Alloys 5083 and 5456 are covered in U.S. Military Specification for armor plate MIL-A-46027E (December 1973, amended May 1975) and MIL-A-46027F (1973, revised June 1976, amended October 1981), all incorporated herein by reference and Aluminum Alloy 7039 in U.S. Military Specification MIL-A-46063E, also incorporated herein by reference. It is generally recognized that for many app-lications AA 7039 armor plate is superior to AA 5083 armor plate, but the advantage is more for armor-piercing ballistic performance and less for fragment simulation performance, at least according to the military specifications. In fact, in thinner gauges AA 5083 armor plate can even sometimes perform better than AA 7039 and AA 7039 can present corrosion or stress corrosion problems to a greater degree than AA 5083 or AA 5456 and is heavier.
The production of 5083 and other 5XXX alloys for armor plate application has, since before the 1970's, normally included hot rolling more than 50% followed by cold rolling to a cold reduction of 10 to 25 or 30%, typically cold rolling about 20%. This was often followed by stretching the cold rolled plate to straighten or flatten it. These practices, as will be appreciated, have been common in the production of various 5XXX-type alloy armor plate products for many years, it being well recognized that hot rolling is a most common way of "breaking down" a large thick ingot into a plate for cold rolling to produce work-hardened Al-Mg alloy plate or sheet products. Cold rolling a 5XXX aluminum alloy produces strain-hardened tempers called H1X tempers, such as H12, H14, H16, with the second digit correlating roughly with the degree of work hardening and strength development. For instance, H14 is stronger than H12, and so on. A third digit is sometimes employed to indicate a special degree of control which does not take the temper outside the characteristics of the first two digits such that H131 is an H13 temper with further or narrower controls to achieve a narrower band of properties which are nonetheless within H13 general type properties. The common cold reductions in prior art (since the 1970's) aluminum 5XXX armor plate (17 to 23%) produced H13 level strength.
While aluminum alloy armor plate, particularly Al-Mg alloy (5XXX alloy) plate, has enjoyed substantial use as armor plate in military vehicles, there remains substantial room for improvement in increased strength and ballistic performance and decreased weight. One such approach exemplified by U.S. Pat. No. 4,469,537 is to very slightly shift the magnesium content upwardly, the method of producing the armor plate being the same as for 5083 and 5456, that is, hot rolling followed by cold rolling a plate, the cold rolling amounting to approximately 20% as was the practice in the prior art.
Substantial increases in magnesium offer benefits of substantially improved strength and ballistics performance along with slightly reduced weight since magnesium is lighter than aluminum. It is recognized, however, that increasing the level of magnesium introduces problems in stress corrosion cracking where significant amounts of cold work are imparted to the sheet or plate product, for instance, amounts of cold work in excess of 10% or 15% as is explained in U.S. Pat. No. 3,708,352, incorporated herein by reference. That patent explains that prior art recognized the corrosion problem in aluminum-magnesium alloys which receive substantial cold work, especially as the magnesium content is increased, and that by eliminating the cold rolling and employing instead warm rolling, the stability of the product was substantially improved such that resistance to stress corrosion cracking was generally acceptable, but resistance to exfoliation was not improved. Thus, in accordance with said U.S. Pat. No. 3,708,352, special thermal controls during or after hot rolling were employed to alleviate the exfoliation problem in the product which was hot and warm rolled.